This invention relates to a feedback control system for controlling the recirculation of exhaust gas through an internal combustion engine, which system has a combustion condition sensor to provide a feedback signal.
Concerning the prevention of air pollution by exhaust gas of internal combustion engines, the recirculation of a portion of exhaust gas back into the engine intake is probably the most widely employed technique for suppressing the emission of NO.sub.x. The recirculation of exhaust gas (EGR) has the effect of lowering the maximum combustion temperature in the engine combustion chambers so that the formation of NO.sub.x in the combustion chambers can be suppressed.
The suppressive effect of EGR on the formation of NO.sub.x is enhanced as the volume of the recirculated exhaust gas relative to the volume of fresh air admitted into the engine is increased (this volume ratio will herein be referred to as EGR rate). To maintain NO.sub.x emission below a permissible level, there is a need of effecting EGR at considerably high EGR rates. On the other hand, the employment of high EGR rates tends to cause instability of the engine operation. Since the recirculated exhaust gas serves as an inert diluent to a combustible gas mixture, not only the maximum combustion temperature but also the combustion pressure lowers as the EGR rate is enhanced. Accordingly the EGR rate should be controlled in dependence on the engine operating condition so as to maintain an adequate balance between the suppression of NO.sub.x emission and the preservation of a stable engine operation, and high precision is required of the control especially when high EGR rates are involved in the scope of the control.
In conventional EGR control systems, it is a usual way of operating an EGR control valve to employ a vacuum-operated actuator which is connected to the induction passage of the engine, so that the EGR control valve is operated in dependence on the magnitude of vacuum either at a venturi section of the induction passage or in the neighborhood of a throttle valve. In this type of control systems, the control is accomplished in a programmed manner so as to regulate the EGR rate to a target value which is preset based on an assumed relationship between the EGR rate or the aforementioned vacuum and the condition of the combustion in the engine.
The venturi section vacuum, for example, is of course an indication of the engine operating condition, but there is a limitation to the precision in the control of EGR when the EGR control valve is directly operated by such vacuum. Furthermore, the rate of EGR in a programmed control system chances to remain constant even though a substantial fluctuation occurs in an actual or realized EGR rate as a result of changes in the engine operating condition. Sometimes the operation of the engine temporarily loses stability from this reason. The instability of the engine operation resulting from a deviation of a realized EGR rate from an intended EGR rate is a matter of great concern particularly when EGR is effected up to a very high EGR rate, for example, about 40%, as in an engine system featuring "two-point ignition" in each combustion chamber, proposed by research workers of Nissan Motor.